1. Field of the Invention
The present invention relates to a boundary acoustic wave device using an SH type boundary acoustic wave and, in particular, the present invention relates to a boundary acoustic wave device including electrodes which are disposed at a boundary between a piezoelectric substance and a dielectric substrate.
2. Description of the Related Art
Previously, various surface acoustic wave devices have been used for RF and IF filters in mobile phones, resonators in VCOs, and VIF filters in televisions. The surface acoustic wave devices utilize a surface acoustic wave, such as a Rayleigh wave or a first leaky wave, propagating along a surface of a medium.
Since the surface acoustic waves propagate along a surface of a medium, the surface acoustic waves are sensitive to changes in the surface condition of the medium. Accordingly, to protect a surface of a medium along which the surface acoustic wave propagates, a surface acoustic wave element is hermetically sealed in a package having a cavity portion such that the surface of the medium described above is disposed therein. Since a package having a cavity as described above has been used, the cost of the surface acoustic wave device is increased. In addition, since the size of the package is larger than the size of a surface acoustic wave element, the size of the surface acoustic wave device is increased.
Another type of acoustic wave is a boundary acoustic wave which propagates along a boundary between solid substances.
For example, in “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and ultrason., Vol. SU-25, No. 6, 1978 IEEE, a boundary acoustic wave device is disclosed in which an IDT is formed on a 126° rotated Y plate X propagating LiTaO3 substrate, and on the IDT and the LiTaO3 substrate, a SiO2 film having a predetermined thickness is formed. In the above technical paper, it has been disclosed that an SV+P type boundary acoustic wave, a so-called Stoneley wave, propagates. In addition, in “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrason., Vol. SU-25, No. 6, 1978 IEEE, the film thickness of the SiO2 film is disclosed as being set to 1.0λ (λ indicates the wavelength of a boundary acoustic wave), and an electromechanical coefficient of 2% is obtained.
The boundary acoustic wave propagates when energy thereof is concentrated at a boundary portion between solid substrates. Accordingly, since the energy is not substantially present on the bottom surface of the LiTaO3 substrate and the surface of the SiO2 film, the properties are not changed by changes in the surface conditions of the substrate and the thin film. Hence, a cavity type package is not required, and as a result, the size of the acoustic wave device is reduced.
In addition, in “Highly Piezoelectric Boundary Acoustic Wave Propagating in Si/SiO2/LiNbO3 Structure” (26th EM symposium, May 1997, pp. 53 to 58), an SH type boundary acoustic wave propagates in a [001]-Si(110)/SiO2/Y-cut X propagating LiNbO3 structure. This SH type boundary acoustic wave has an increased electromechanical coefficient k2 as compared to that of the Stoneley wave. In addition, with the SH type boundary acoustic wave, as with the Stoneley wave, the cavity type package is not required. Furthermore, since the SH type boundary acoustic wave is an SH type wave, the reflection coefficient of strips forming an IDT reflector is increased as compared to that of the Stoneley wave. Hence, for example, when a resonator or a resonator filter is formed using the SH type boundary acoustic wave, miniaturization is achieved, and in addition, steeper properties are obtained.
In a boundary acoustic wave device, a large electromechanical coefficient is required. In addition, small propagation loss, power flow angle, and temperature coefficient of frequency are also required. The propagation loss degrades the insertion loss of a boundary acoustic wave filter or degrades an impedance ratio of a boundary acoustic resonator. The impedance ratio is a ratio between a resonant resistance or the impedance at a resonant frequency and the impedance at an antiresonant frequency. Accordingly, reduced propagation loss is preferable.
The power flow angle is an angle showing the difference in direction between the phase velocity of a boundary acoustic wave and the group velocity of energy thereof. When the power flow angle is large, an IDT must be obliquely arranged in accordance with the power flow angle. As a result, designing the electrodes is complicated. In addition, due to the deviation in angle, the generation of loss is more likely to occur.
Furthermore, when the operating frequency of a boundary acoustic wave device is changed by temperature, in a boundary acoustic wave filter, practical passband and stopband regions are decreased. In the case of a resonator, the change in operating frequency by temperature described above causes abnormal oscillation when an oscillation circuit is formed. Hence, the change in frequency per degree centigrade, i.e., TCF, is preferably reduced.
For example, when reflectors are provided along a propagation direction and outside a region in which a transmitting and a receiving IDT, which respectively transmits and receives a boundary acoustic wave, are provided, a resonant filter having a low loss is produced. The band width of this resonant filter depends on the electromechanical coefficient of the boundary acoustic wave. When the electromechanical coefficient k2 is large, a wide band filter is obtained, and when it is small, a narrow band filter is obtained. Accordingly, the electromechanical coefficient k2 of a boundary acoustic wave which is used for a boundary acoustic wave device must have a value that is appropriate for its application. When an RF filter for mobile phones is produced, the electromechanical coefficient k2 must be at least 5%.
However, in the boundary acoustic wave device using a Stoneley wave, as disclosed in “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrason., Vol. SU-25, No. 6, 1978 IEEE, the electromechanical coefficient k2 is only about 2%.
In addition, in the Si/SiO2/LiNbO3 Structure disclosed in “Highly Piezoelectric Boundary Wave Propagating in Si/SiO2/LiNbO3 Structure” (26th EM symposium, May 1997, pp. 53 to 58), in order to excite the boundary acoustic wave, as shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 10-84247, a complicated four-layered structure of Si/SiO2/IDT/LiNbO3 was required. Furthermore, when Si was arranged in the [001]-Si(110) orientation which was proposed as the most optimal conditions, a highly difficult bonding method had to be used as disclosed in Japanese Unexamined Patent Application Publication No. 10-84247. In general, it has been difficult to uniformly bond a wafer having a diameter of 3 inches or more, which is used for mass production, by a bonding method. In addition, when the wafer was cut into chips after bonding, defects such as peeling often occur.
With respect to the SH type boundary acoustic wave, as disclosed in “Investigation of Piezoelectric SH Type Boundary Acoustic Wave”, Technical Report, The Institute of Electronics, Information and Communication Engineers, Vol. 96, No. 249 (US96 45-53) PAGE. 21 to 26, 1966, in the structure including an isotropic substance and a BGSW substrate, when the conditions are satisfied such that the acoustic velocity of a transverse wave of the isotropic substance and that of a transverse wave of the BGSW substrate are close to each other, the density ratio is small, and the piezoelectric properties are strong, the SH type boundary acoustic wave is obtained.
However, due to limitation of materials that are capable of satisfying the conditions described above, it is difficult to satisfy the aforementioned conditions and properties required for the boundary acoustic wave. For example, in the [001]-Si(110)/X—LiNbO3 structure disclosed in “Highly Piezoelectric Boundary Wave Propagating in Si/SiO2/LiNbO3 Structure” (26th EM symposium, May 1997, pp. 53 to 58), it is necessary to use a highly difficult bonding method for production.